Ultrasonic imaging of the cornea and lens capsule presents a problem not generally encountered in other types of tissue. The corneal and lens capsule surfaces are necessarily smoothly curved in order to perform their optical function of focusing light rays. Because the cornea and lens structures are smooth and regular, ultrasonic energy is reflected directly back to that transducer when the beam is aligned perpendicular to the corneal or lens surface. This kind of reflective property is call specular reflection. Because of the specular property of corneal and lens surfaces, it will be appreciated that special care must be taken to align the transducer so that the emitted acoustic pulses impinge normally on the surface of the cornea or the lens surface of interest at each position from which an element of the image is to be formed.
In the case of the cornea, ultrasonic imaging of large portions of the cornea can be accomplished by scanning the transducer along the corneal surface while continually adjusting the alignment of the transducer to provide a beam that is always directed approximately through the cornea's center of curvature. Corneal imaging and measurement of corneal dimensions require that the scanning motion of the transducer be smooth and precisely aligned. Departures of the transducer position as small as 5 microns from a circular path or of the beam's direction from the center of curvature can significantly degrade the resulting image. Mechanisms for performing the requisite scan alignment are described in U.S. Pat. Nos. 5,331,962; 6,887,203; and U.S. patent application Ser. No. 12/347,674.
While ultrasonic imaging may be used by ophthalmologists for quantitative analysis of laser refractive surgery, it may also be used for implantation of corneal and phakic lenses, implantation of intraocular lenses including accommodative lenses and specialty procedures such as glaucoma and cataract treatment. The use of ultrasonic imaging of important features of the eye for lens implantation is discussed, for example, in U.S. Pat. No. 7,048,690. This patent does not include techniques for imaging the posterior surface of the lens capsule and so cannot be used to compute the volume of a lens capsule.
The components of the eye in front of the iris can be mapped and measured by both optical and acoustic means to a high degree of accuracy. Except for features near the optical axis, the components of the eye behind the iris cannot be imaged optically but can be imaged by acoustic techniques. In particular, the region of the lens capsule where the anterior and posterior surfaces meet and the zonules that attach the lens capsule to the ciliary body cannot be observed by optical systems but can be successfully imaged by high frequency (10 to 50 MHz) ultrasound devices. New procedures such as implantation of accommodative lenses may provide nearly perfect vision without spectacles or contact lenses. Implantation of accommodative lenses, if full accommodation is to have a high degree of success, requires precision measurements of, for example, the width, volume and shape of the natural lens capsule for successful lens implantation. Ultrasonic imaging can be used to provide the required accurate images of the natural lens especially where the zonules attach the lens to the ciliary muscle, all of which are well off-axis and behind the iris and therefore not accessible to optical imaging.
Ultrasonic imaging has other advantages over optical imaging devices, even in the cornea. For example, ultrasonic pulses interact on the basis of acoustic/mechanical properties with the various components of the eye whereas optical pulses may interact weakly since the eye has evolved primarily to refract but not substantially reflect or attenuate. These differing physics give rise to another advantage for acoustic imaging. Corrections due to differing acoustic velocities of the various eye components are on the order of about 1 to 10 percent whereas optical corrections for index of refraction differences are on the order of about 25 to 35 percent. This gives ultrasonic imaging an advantage for measuring such features as the angle between the iris and corneal sclera in the anterior chamber.
It must be appreciated that ultrasonic imaging requires a liquid medium to be interposed between the object being imaged and the transducer, which requires in turn that the eye, the transducer, and the path between them be at all times be immersed in a liquid medium. Concern for safety of the cornea introduces the practical requirement that the liquid medium be pure water or normal saline water solution. In either case, the mechanism or major portions of it must be submerged in water for long periods.
Typically, ultrasonic imaging practice uses a single fixed transducer which can move along an arcuate guide for both sending ultrasound pulses to and receiving echos from eye structures. It is readily demonstrated that, with this arrangement, specular surfaces only return echos along the axis of the incident beam if the incident beam is directed normally or perpendicularly to the surface. This behavior has led to the development of ultrasound imaging devices that maintain their incident beam approximately perpendicular to the corneal or lens surface as the incident ultrasound beam scans the surface. An exemplary embodiment of such a device is described in U.S. patent application Ser. No. 12/347,674 filed Dec. 31, 2008 entitled “Components for an Ultrasonic Arc Scanning Apparatus” and in U.S. patent application Ser. No. 12/418,392 filed Apr. 3, 2009 entitled “Procedures for an Ultrasonic Arc Scanning Apparatus ”, both of which are incorporated herein by reference. With such a device, the incident beam scans in a plane while directing its axis through a fixed center of curvature. If that center of curvature of the arcuate guide is coincident or nearly coincident with the center of curvature of the corneal or lens surface, the incident beam will remain approximately perpendicular to the surface throughout the scan, and ultrasound reflections will be returned to the transducer from all scanned parts of the surface, allowing the formation of a complete image.
While all of the corneal surfaces have nearly the same center of curvature locally, the center of curvature may vary along the arc of the cornea. That is, the entire cornea typically does not form a perfectly circular arc. Moreover, the natural lens anterior and posterior surfaces are centered at different locations from one another and from that of the cornea. The lens surfaces also typically do not form perfectly circular arcs.
There remains, therefore, a need for an ultrasonic arc scanning method that can produce a superior and more comprehensive image of a cornea, lens and other eye components, such as the zonules that attache the lens capsule to the ciliary body, than an arc scanner with a fixed focal point such as described, for example, in U.S. patent application Ser. No. 12/418,392.